1. Field of the Invention
The present invention relates to a filter circuit used for limiting a radio band of radio communications and a radio communication apparatus using the same.
2. Description of the Related Art
Generally, a filter circuit device comprises plural resonators connected in cascade. The resonator is configured with inductors and a capacitor. In the case that the effect of a loss is considered, a resistor is added to the resonator. The resonance frequency of the resonator when no resistor is provided is expressed by the following equation.f0=1/sqrt(L×C)
where L and C indicate an inductance and capacitance of the resonator respectively. It is possible to determine a pass frequency band and a decay quantity of the filter circuit by connecting the resonators in cascade and determining adequately coupling factors (m2, m3) representing a coupling quantity between the resonators and external Qs (m1, m4) representing a quantity by which the resonator excites an input/output port.
A real filter circuit comprises a filter circuit using as a resonator a three-dimensional circuit such as a filter configured with a metal cavity or a filter configured with a cylindrical metal cavity in which a dielectric material is inserted. Alternatively, it comprises a filter circuit using a distributed constant circuit such as a filter configured with a microstrip line or a resonator of a plane circuit or a lumped constant circuit configured with circuit constants such as an inductor or a capacitor. There is a filter using a microstrip line resonator as an example of the filter. This filter uses three microstrip line resonators of a half-wave length, which are arranged such that their outputs are shifted by a quarter-wave. The distance between the resonators determines the coupling factor between the resonators.
The excitation lines on the input and output sides are arranged at a distance realizing a desired external Q with respect to the resonator. Many of these filters each comprise plural resonators all connected in cascade. Substantially the same electric energy passes through all resonators. However, the electric energies passing through the resonators slightly differ due to respective losses contained in the resonators. Therefore, it is important that a filter passing through a high electric energy has a structure for radiating heat due to the loss of the resonator. The filter of high energy resistance performance has a large size, and uses a filter using a three-dimensional circuit which is excellent in low loss characteristics and radiation characteristics. Conventionally, the filter size can be decreased in order of a three-dimensional circuit, a distributed constant circuit and a lumped constant circuit. However, there is a problem that a loss increases and a heat radiation characteristic deteriorates.
There is a method of configuring a filter of lower loss than the three-dimensional circuit with a microstrip line filter using a superconductor to realize a low loss and a small size. The filter configured with microstrip line resonators connected in cascade, each resonator having a length of a half-wave length of a desired frequency, is known (Takayuki Kato, Kenji Yamanaka, Zhewang Ma, Yoshio Kobayashi, “Studies on the equivalent circuits of dual-mode rectangular waveguide filters using HFSS and MDS” Faculty of Engineering, Saitama University, MW 98-85, pp. 73-80, Sep. 1998). However, in the microstrip line resonator, an electric field concentrates on the sectional edge of the line through which a signal power passes, so that an electric current concentrates thereon. For this reason, there is a problem that, if the high power passes through the filter, the current flowing through the edge with the power of several watts exceeds a limiting value of the critical current density of the superconductor, resulting in damaging the superconducting characteristic.
A filter configured with resonators connected in parallel in order to reduce heat radiation for the filter using the three-dimensional circuit is known (Japanese Patent Laid-Open No. 2001-345601). A filter improving a power handling capability as a whole is realized by distributing a power supplied by a parallel structure of resonators to each resonator. If the resonators are configured to have different frequencies for realizing a parallel structure of the resonators and the resonators having adjacent resonance frequencies are configured so as to have a reversed phase, a filter with a desired filter property can be realized. However, it becomes difficult to make the resonators different in resonance frequency with the three-dimensional circuit to realize such a filter.
To perform detection in reversed phase with the three-dimensional circuit can be realized by carrying out detection in an electric field mode for making the reversed phase or by reversing a direction of a loop antenna for detecting a magnetic field. However, it is impossible to perform detection in reversed phase in a case of using the distributed constant circuit and lumped constant circuit. Therefore, a filter structure becomes large in size when the resonators are connected in parallel. Further, if the resonators are configured with microstrip lines and connected in parallel, a set of a resonator and a delay line more than 180 degrees is needed, thereby to increase a circuit scale.
As mentioned above, in a conventional filter configured with resonators connected in cascade, when a high electric energy is supplied to a filter, the high electric energy passes through all resonators. As a result, it is difficult to obtain a high power handling capability. In particular, in the filter using microstrip line resonators, when the high electric energy passes through the filter, a current concentrates at edge of the signal line. As a result, the concentrated current exceeds the critical current density of the superconductor, resulting in damaging the superconducting characteristic. Further, a delay circuit for realizing an reversed phase increases in size for the resonators to be connected in parallel.
An object of the present invention is to provide a filter circuit capable of decreasing in size by connecting resonators in parallel even if a resonance circuit of a distributed constant circuit or a lumped constant circuit is used.